The present invention relates to the use of a probe for the control of foam build up in a fermenter, to a sensor comprising the probe for use with an antifoam supply regulator in the control of foam build up in a fermenter, to such a foam control system, to a fermenter comprising the foam control system, and to a method of controlling foam build up in a fermenter.
During many microbiological fermentation processes foam is produced. Its presence is undesirable since it reduces the amount of oxygen reaching the broth during the fermentation and impedes the processing of the broth after the fermentation has been completed. Furthermore, if uncontrolled, it can block exit gas lines and eventually spill out of the fermenter.
Much attention has been devoted to the development of automatic control systems for the prevention of foam build up in fermenters. Such systems typically comprise a sensor which detects when foam has risen up to a predetermined level, and an antifoam dispenser which supplies an antifoam substance to the fermenter when the sensor detects the foam. Suitable antifoam substances include polypropylene glycol 2,000 and silicone based products, for example Dow Corning silicone RID emulsion.
One known and commercially available type of foam sensor comprises a probe having an electrode which, in use, is inserted through a wall or closure in the fermenter to the predetermined level and powered with a low voltage. When foam has risen up to touch the electrode, an electric current begins to flow through the foam and fermentation broth to earth. Thus the potential at the electrode drops, and this change in potential is exploited to activate an antifoam supply. However, it is found that such probes are unreliable over a long period of time. Thus foam control systems comprising this known type of probe tend to deliver increasingly excessive amounts of antifoam substance to the fermentation broth as the fermentation progresses.
The presence of excess antifoam substance in a fermentation broth is disadvantageous because it reduces the rate of inward diffusion of oxygen and makes downstream processing, particularly product recovery, difficult. Clearly, therefore, there is a need for a probe capable of being used in the control of foam produced during fermentation without resulting in unacceptably high levels of antifoam substance in the fermentation broth.
The specification of U.S. Pat. No. 3,119,266 (US-A-3,119,266) discloses a probe for detecting or measuring the level or quantity of liquid, powder, granular or aggregate materials in bins, tanks or other containers, which comprise first and second, or measuring conductive electrodes and a guard electrode which is driven in phase with the voltage derived between the measuring electrodes with a voltage of substantially the same magnitude as the measuring voltage.
The specification of U.S. Pat. No. 4,027,172 (US-A-4,027,172) discloses a modified version of the probe described in US-A-3,119,266, with particular emphasis placed on the use of the modified probe in detecting the presence or absence of boiler water in heating plant boilers. The features of the probes of both US-A-3,119,266 and US-A-4,027,172 are combined in the specification of U.S. Pat. No. 4,390,793, which discloses a multifunction electronic probe circuit for water level control.